An electric vehicle obtains its driving energy not from combustion of fossil fuel like existing vehicles but from electric energy. The electric vehicle has advantages of substantially no exhaust gas and very low noise, but the electric vehicle has been not put to practical use since its battery is too heavy and it takes a long time for charging the battery. However, as serious environmental problems and exhaustion of fossil fuels recently become important issues, the development of electric vehicles is accelerated again. In particular, in order to practically use electric vehicles, a battery pack serving as a power source of an electric vehicle should be lighter and smaller, and the time required for charging the battery should be more shortened, so the battery pack is being actively studied.
The battery pack includes a plurality of battery cells connected in series, and heat is generated from the battery cells when the battery pack is charged or discharged. If the heat generated from the battery cells is neglected, the life span of the battery cells is shortened. Thus, in common cases, the battery pack has a cooling channel for eliminating the heat generated from the battery cells.
The battery pack may be classified into Z-type battery packs and U-type battery packs depending on the shape of the cooling channel. In the Z-type battery pack, air serving as a cooling gas is introduced to and discharged from the cooling channel in the same direction. In the U-type battery pack, the direction in which air serving as a cooling gas is introduced to the cooling channel is opposite to the direction in which the air is discharged from the cooling channel. Hereinafter, a conventional Z-type battery pack is explained with reference to FIGS. 1 and 2. FIG. 1 is a perspective view showing a conventional Z-type battery pack, and FIG. 2 is a sectional view taken along the line A-A′ of FIG. 1.
The Z-type battery pack 10 includes a plurality of battery cells 20 connected in series and cooling channels 30, 40 coupled to the battery cells 20. The cooling channels 30, include a first cooling channel 30 coupled to a top of the battery cell 20 and a second cooling channel 40 coupled to a bottom of the battery cell 20.
One side 32 of the first cooling channel 30 is open such that a cooling gas may be introduced therein. Also, a plurality of slits 34 are formed in a lower surface of the first cooling channel 30, not coupled with the battery cells 20, such that the introduced cooling gas may be discharged toward the battery cells 20.
A plurality of slits 44 are formed in an upper surface of the second cooling channel 40, not coupled with the battery cells 20, such that the cooling gas discharged from the first cooling channel 30 is introduced. Also, one side 42 of the second cooling channel 40 is open such that the cooling gas introduced through the slits 44 may be discharged to the outside.
The cooling gas introduced through the side 32 is discharged to the outside while passing through the slits 34, the spaces between the battery cells 20, and the slits 44 in order. In this procedure, the cooling gas absorbs heat from the battery cells 20, so the battery cells 20 may be cooled.
However, in the battery pack 10, as being apart from the side 32 farther, the difference of pressures between the first cooling channel 30 and the second cooling channel 40 is increased. Thus, among the spaces between the battery cells 20, any space located farther from the side 32 allows a larger amount of cooling gas to pass. This phenomenon causes some problems. In other words, among the battery cells 20, battery cells located near the side 32 may be not sufficiently cooled, and temperature variation of the battery cells 20 is too great, as shown in FIG. 3.
FIG. 3 is a graph showing temperature distribution of the battery cells 20 included in the battery pack 10. In FIG. 3, a horizontal axis represents the number of each battery cell 20, and a vertical axis represents temperature of each battery cell 20. The number of battery cell 20 is set to increase in a right direction based on FIG. 2. Also, the results shown in FIG. 3 are derived under the condition that loads of 40A are applied to a battery pack 10 including 44 battery cells 20 with an ambient air of 30° C.
Seeing FIG. 3, it would be found that the temperature of battery cells over the number of 28 exceeds 50° C. In this case, the battery cell cannot be used for a long time, and the life of battery cell is rapidly reduced. Even worse, the battery cell may be exploded.
In addition, seeing FIG. 3, a difference between the maximum temperature and the minimum temperature of battery cells is about 35° C. In this case, life spans of battery cells are seriously different. If just some of the battery cells included in the battery pack run out, the entire battery pack should be exchanged, so other battery cells having a residual life cannot be used due to some run-out battery cells.